In many applications, particulate binders are needed to hold components together for improved processability, mechanical stability, adhesion properties and to enhance performance of the components. For example, U.S. Pat. Nos. 4,317,778, 4,351,773 and 4,374,043 describe the production of vanadium phosphate (VPO) catalysts for hydrocarbon oxidation in which silica sol is used to bind VPO particles to increase attrition resistance of the catalysts.
In certain surface coating and printing applications, particulate binding agents are used to enhance the mechanical stability of a coating or printing element and the adhesion of the element to the substrate surface.
Silica (SiO2) is typically used for these applications rather than titanium dioxide. On reason may be that silica particles are normally amorphous whereas titanium dioxide is typically a crystalline material. Amorphous particles characteristically contain a high concentration of hydroxyl groups on the surface, which provide a large number of active sites on the surface of the particles, resulting in strong binding to other surfaces. In contrast, titanium dioxide is typically present as a crystalline solid with lower binding properties.
However, there are advantages to using titanium dioxide rather than silica in binding applications. For example, titanium dioxide is more stable chemically than silica, and titanium dioxide is photoactive and is more active catalytically. For this reason, titanium dioxide is used for surface coating applications where photocatalytic properties are useful.
Titanium dioxide is typically produced commercially in two crystal phases, rutile and anatase, that differ in lattice structures, refractive indices, and densities. The rutile phase is the more stable phase and is favored for use in pigment applications because rutile pigments have a higher refractive index than their anatase counterparts, resulting in greater opacity and whiteness.
The anatase form of titanium dioxide is usually more photoactive than the rutile form and is typically used for photocatalytic applications. When exposed to light, the photoactive titanium dioxide forms reactive species which can degrade NOx and volatile organic compounds (VOCs) that come into contact with the photocatalytic material. In view of these properties, photocatalytic titanium dioxide has been employed in coatings to remove pollutants from the environment. Such coatings may also have the advantage of being self-cleaning since soil (grease, mildew, mold, algae, etc.) is also oxidized on the surface.
U.S. Patent Application Publication No. US 2004/0241502 to Chung et al., which is hereby incorporated by reference, describes neutral and transparent titanium dioxide colloid sols and a method of manufacturing the sols. The publication discloses that the titanium dioxide particles used in the sols is in the anatase form. The sols described contain 1-5% titanium dioxide by weight of the mixture.
U.S. Pat. No. 6,824,826 to Amadelli et al., which is hereby incorporated by reference, describes the preparation of colloidal TiO2 which may be doped with a metal chosen from the group I-VA, and the lanthanide and actinide series for coating on cementitious stone and marble products. The colloidal titanium dioxide is produced by hydrolysis of a TiO2 precursor in dilute nitric acid.
U.S. Pat. No. 6,627,336 to Ohmori et al., which is hereby incorporated by reference, describes an aqueous dispersion of finely divided titanium dioxide comprising chloride ion and at least one Bronsted base selected from pyrophosphate, metaphosphate, polyphosphate, methanesulfonate, ethanesulfonate, dodecylbenzenesulfonate and propanesulfonate.
U.S. Pat. No. 6,737,485 to St. Clair et al., which is hereby incorporated by reference, describes stabilized dispersions formed from titanium chelate polymer and a process to prepare the stabilized dispersions of titanium chelate polymer. The dispersion is stabilized by various stabilizing compounds including hydroxyacids.
U.S. Pat. No. 6,420,437 to More et al., which is hereby incorporated by reference, discloses neutral titanium dioxide colloid sol said to have high stability in a neutral pH range and is capable of forming a colorless transparent coating.
Therefore, there exists a need for stable, translucent or transparent titanium dioxide sols comprising amorphous titanium dioxide that will combine the superior stability and photocatalytic activity of titanium dioxide with the desirable binding properties of an amorphous solid.
The foregoing discussion is presented solely to provide a better understanding of nature of the problems confronting the art and should not be construed in any way as an admission as to prior art nor should the citation of any reference herein be construed as an admission that such reference constitutes “prior art” to the instant application.